Digital photos are easy to zoom in certain zoom ratios. Some examples of these are presented in FIGS. 8a and 8b. These ratios may form a special series that is used as a default when performing zooming in the selected zooming direction. Some examples of these default or user interface ratios, depending on term choice to be used, are 2n:1 (2:1, 4:1, 8:1, etc), 1:1, and 1:2n (1:2, 1:4, 1:8, etc) ratios. These set of ratios are easy because a group of pixels can be easily shrunk into one pixel simply by calculating their average (zoom-out, FIG. 8a), or enlarged simply by duplicating the pixel (zoom-in, FIG. 8b). These zoom ratios also maintain the visible quality of the image. These zoom ratios are called the “easy” or default or user interface zoom ratios in connection with this document.
If some other zoom ratios are used (e.g., 1:3) some problems arise. The user is either going to see jagged edges if nearest neighbour interpolation is used. On the otherwise the software is going to have to use CPU-intensive calculations to achieve high visual quality if bicubic interpolation is used. In general, the different interpolation methods are quite well known for the one versed in the art.
However, since device screens contains a fixed number of pixels in each screen directions, a device will often have to use some other zoom ratio (than mentioned above) to fill the entire screen. A basic example of this situation is described in FIGS. 3 and 4b. For example, when filling a screen 11 having 176×208 pixels with an 640×480 original image IM(1:1), the device will have to use a zoom ratio of 27,5% (left side part of FIG. 4b). From this fit-to-screen zoom ratio ZF, many devices (or software) allow user to zoom further into the image (right side part of FIG. 4b). It makes sense to use the “easy” zoom ratios as default zoom ratios when zooming due to the reasons mentioned above (quality and processing capability). So, if the user zooms in from 27,5% (=ZF), the device would pick the next zoom ratio (=ZRN) of the set of the “easy” zoom ratios, i.e., 50% (1:2).
However, sometimes when beginning from the fit-to-screen zoom ratio ZF the zoom increase to the next “easy” zoom ratio ZRN is very small. This prior art example is described in FIG. 7. Due to this reason the user is not going to see much difference between zoomed images IM(ZF) and IM(ZRN) (=image objects IO). The next “easy” zoom ratio ZRN was used, regardless of whether it offered very little visible difference to the user. So zoom, for example, in from ZF=11% (left side part of FIG. 7) goes to ZRN=12,5% (right side part of FIG. 7). However, the user will have to wait for the zooming to be completed. This is very frustrating in the view of user-friendliness because this takes time and, however, the result between the viewed images IM(ZF) and IM(ZRN) is not so noticeable on the screen 11 due to the small zooming effect.